The present invention relates to a circuit board excelling in connection reliability and wiring-pattern adhesive strength, and to a method for producing the same.
Recently, following the tendency of electronic devices toward compact size, less weight, improved function, and improved performance, it is increasingly demanded, not only in industrial equipment fields but also widely in consumer equipment fields, to supply inexpensive multi-layer circuit boards on which semiconductor chips such as LSIs can be mounted at a high density.
In response to the demand of the market, a technology has been developed whereby, in place of a conventional ceramic multi-layer board, a resin multi-layer circuit board available at a lower price is rendered suitable for high-density mounting.
A circuit board of xe2x80x9cALIVHxe2x80x9d (a trademark of Mastushita Electric Industrial Co., Ltd.) structure, that is, a circuit board having an inner via hole structure for all layers, for instance, has been proposed as the above circuit board (JP6(1994)-268345A). This circuit board can be supplied at a relatively low price since it is a resin multi-layer board in which insulator layers are made of a composite material of a non-woven aramid fabric reinforcer and an epoxy resin. It is also suitable for high-density mounting since the inner-via-hole connection technique permits arbitrary portions of wiring layers to be connected via a conductive paste, that is, the xe2x80x9cALIVHxe2x80x9d structure is applied.
In such a circuit board, it is important to secure the reliability of the inner-via-hole connection, but the reliability of inner-via-hole connection has had problems as described below.
Specifically, inner-via-hole connectors are made of a different component material from that of an insulator layer, thereby having different physical properties (thermal expansion coefficient, humidity expansion coefficient, etc.) from those of the insulator layer. Therefore, because of the mismatch between an organic resin material for the insulator layer that has a relatively great thermal expansion coefficient and an inner-via-hole connector material containing a metal with a relatively small thermal expansion coefficient as a main component, there particularly has been a drawback in that stress in a thickness direction occurs inside the inner-via-hole connector material due to a heat cycle. When an excessive stress is applied, IHV connectors are damaged, thereby causing the connection reliability to deteriorate.
A technique that seems to be applicable for solving this problem is a technique of decreasing the difference between the physical properties of materials forming the insulator layer and the inner-via-hole connectors so as to prevent the damage to the IHV connectors. To be more specific, by adding an inorganic filler such as silica to a resin of the insulator layer so as to decrease the content of the resin in the insulator layer, the thermal expansion coefficient of the insulator layer is decreased.
The resin of the insulator layer, however, functions for firmly bonding the wiring layer to the insulator layer. In the case where the insulator layer includes a decreased content of the resin in surface part, it loses its adhesive strength, and problems arise such as exfoliation of the wiring layer, exfoliation of components mounted on a component-mount surface together with pads, etc. Thus, the aforementioned technique does not suffice.
The present invention has been made to solve the aforementioned problems in the prior art, and it is an object of the present invention to provide a circuit board that exhibits sufficient bond strength between the wiring layer and the insulator layer and that provides excellent inner-via-hole connection reliability, as well as to provide a method for producing the circuit board.
To achieve the above object, a circuit board of the present invention includes:
not less than two wiring layers,
an insulator layer for electric insulation between the wiring layers; and
an inner-via-hole conductive member provided in the insulator layer in a thickness direction of the insulator layer for electric connection between the wiring layers,
wherein:
the insulator layer is made of a composite material containing an organic resin and a material having a smaller thermal expansion coefficient than that of the organic resin, and includes a surface part, a core part, and a surface part laminated in the stated order, the surface part having a relatively high content of the organic resin, the core part having a relatively low content of the organic resin;
the wiring layers have a land portion that is connected with the inner-via-hole conductive member, the land portion being embedded so as to be substantially in contact with the core part; and
the inner-via-hole conductive member has a thickness substantially equal to a thickness of the core part.
A first circuit board production method of the present invention is a method for producing a circuit board that includes not less than two wiring layers, an insulator layer for electric insulation between the wiring layers, and an inner-via-hole conductive member provided in the insulator layer in a thickness direction of the insulator layer for electric connection between the wiring layers, and
the circuit board producing method comprises the steps of:
making an insulator layer of a composite material containing an organic resin and a material having a smaller thermal expansion coefficient than that of the organic resin so that the insulator layer includes a surface part, a core part, and a surface part laminated in the stated order, the surface part having a relatively high content of the organic resin, the core part having a relatively low content of the organic resin;
opening an inner via hole in the insulator layer in a thickness direction of the insulator layer, and filling the inner via hole with a conductive material;
providing metal foils on both sides of the insulator layer, each metal foil having a protruded portion at a predetermined position so that the position coincides with a position of the inner via hole filled with the conductive material;
applying heat and pressure to external sides of the metal foils by hot pressing so that the protruded portions of the metal foils come substantially in contact with the core part of the insulator layer, to cause the inner via hole filled with the conductive material to form an inner-via-hole conductive member with a thickness substantially equal to a thickness of the core part; and
etching the metal foils so as to leave the protruded portion as the land portion.
A second circuit board producing method of the present invention is a method for producing a circuit board including not less than two wiring layers, an insulator layer for electric insulation between the wiring layers, and an inner-via-hole conductive member provided in the insulator layer in a thickness direction of the insulator layer, for electric connection between the wiring layers, and
the circuit board producing method comprises the steps of:
making an insulator layer of a composite material containing an organic resin and a material having a smaller thermal expansion coefficient than that of the organic resin so that the insulator layer includes a surface part, a core part, and a surface part laminated in the stated order, the surface part having a relatively high content of the organic resin, the core part having a relatively low content of the organic resin;
opening an inner via hole in the insulator layer in a thickness direction of the insulator layer, and filling the inner via hole with a conductive material;
providing transfer bases on both sides of the insulator layer, each of the transfer bases having a metal foil patterned in a predetermined pattern so as to coincide with a position of the inner via hole filled with the conductive material; and
applying heat and pressure to external sides of the transfer bases by hot pressing so that the metal foils come substantially in contact with the core part of the insulator layer, to cause the inner via hole filled with the conductive material to form an inner-via-hole conductive member with a thickness substantially equal to a thickness of the core part.
In the above, xe2x80x9cthe inner-via-hole conductive member has a thickness substantially equal to a thickness of the core partxe2x80x9d means that the thickness of the inner-via-hole conductive member is equal to the thickness of the core part, or not-more-than-10% greater than the thickness of the core part, or not-more-than-5-xcexcm greater than the thickness of the core part.
According to the present invention, the wiring layers are held by an organic resin excelling in adhesiveness, while mismatch hardly occurs between the thermal expansion coefficients. Therefore, it is possible to provide a circuit board that exhibits sufficient bond strength between the wiring layer and the insulator layer, and that provides excellent inner-via-hole connection reliability, as well as to provide a method for producing the circuit board. Furthermore, since a part of the metal foil is embedded in the insulator layer so as to be in contact with the core layer thereof, portions of the conductive material can be selectively compressed, and hence this makes it possible to provide a circuit board capable of ensuring stable connection between layers.